Modern audio coding methods such as e.g. MPEG Layer 3, MPEG AAC or MPEG HE-AAC are capable of reducing the data rate of digital audio signals by means of exploiting psycho-acoustical properties of the human ear. Hereby a block of a fixed number of audio samples, called frame, is transformed in the frequency domain. Adjacent frequency coefficients are grouped together into scalefactor bands. The coefficients of each scalefactor band are quantized and the quantized coefficients are entropy coded into a compressed bitstream representation of this frame. The quantization step size is controllable for each individual scalefactor band. It has to be chosen such that on the one hand the resulting quantization noise is smaller than a threshold given by the perceptual model of the encoder, but on the other hand that the number of bits necessitated for encoding this scalefactor band is as small as possible. These are two contrary conditions: Reducing the quantization noise is normally accomplished by decreasing the quantization step size of the quantizer, resulting in larger quantized values. Entropy coding schemes as e.g. Huffman coding for MPEG Layer 3 or MPEG AAC of the quantized values are usually designed to spend less bits on the smaller values because of the greater occurrence of small quantized values. Since the spectral coefficients are signed, all quantized coefficients except for the quantization index 0 need one bit in addition to store the sign.
Quantizers in conventional methods are usually designed in such a way that the resulting quantization error will be minimized. However it is not considered that the bit demand for different quantized values is not equal.